1. Field of the Invention
The present invention relates to a printing apparatus for printing images by selectively feeding inks and water to a plurality of image areas on a plate mounted on a plate cylinder.
2. Description of the Related Art
A printing apparatus is known which includes a plate cylinder for supporting a plurality of plates mounted peripherally thereof, and a plurality of ink feeders for feeding different color inks to image areas on the plurality of plates mounted on the plate cylinder, respectively. This printing apparatus prints images with a plurality of color inks by using the single plate cylinder and a single impression cylinder.
Japanese Patent Publication (Unexamined) H3-143634 (1991), for example, describes a printing apparatus for printing images on printing paper with two color inks. This apparatus includes a plate cylinder for supporting two plates mounted peripherally thereof, two ink feeders for feeding the different inks to the respective plates on the plate cylinder, a single dampening water feeder for feeding dampening water to the respective plates on the plate cylinder, and an impression cylinder having half a diameter of the plate cylinder. A sheet of printing paper is fed for every two rotations of the impression cylinder.
Each ink feeder in the printing apparatus described in the above publication has an ink applying roller for applying ink to one of the plates. The ink applying roller is vertically movable by a cam mechanism synchronously with rotation of the plate cylinder, whereby the ink applying roller is placed in contact with only the corresponding plate.
In each ink feeder used in the above printing apparatus, when the single ink applying roller is placed in contact with the surface of the plate to feed ink thereto, the ink tends to be fed to the plate in an insufficient quantity and tends to be applied unevenly over the plate. To avoid such inconveniences, it is desirable to arrange a plurality of ink applying rollers circumferentially of the plate cylinder.
When vertically moved en bloc, these ink applying rollers will contact different positions of the plate. Consequently, the ink may be fed in different quantities to different positions of the image areas on the plate. Further, an operation for feeding the ink from the ink applying rollers to the plate is carried out in parallel with a printing operation. When the ink applying rollers are vertically moved en bloc, vibrations caused by the vertical movement will be transmitted to the plate cylinder, thereby impairing printing precision.
In the multicolor printing apparatus described in Patent Publication (Unexamined) H3-143634 (1991), the single dampening water feeder feeds dampening water to the plates to be printed with different color inks. With such a construction, however, the inks adhere in emulsified state to a roller such as a water applying roller of the dampening water feeder. Then, inks of other colors than the intended color inks are applied to the image areas on the plates. This gives rise to a problem of smudging prints.
In the multicolor printing apparatus noted above, two plates are mounted peripherally of the single plate cylinder. The plate cylinder is complicated in construction since grippers must be arranged peripherally thereof for holding the forward end and rear end of each of the two plates. Moreover, a plate mounting operation is time-consuming in attaching two plates to the outer periphery of the plate cylinder.
These drawbacks may be overcome by attaching a single plate to the periphery of the plate cylinder, and forming a first and a second image areas on the plate to be printed with different color inks. In this case, when feeding the different color inks from the ink feeders to the first and second image areas, it is necessary to prevent the inks from adhering to an area between the first and second image areas on the plate.